The present invention relates generally to power supply systems, and more specifically to power supply systems including flowing electrolyte batteries.
Batteries used in certain prior art stand alone power supply systems are commonly lead-acid batteries. However, lead-acid batteries have limitations in terms of performance and environmental safety. Typical lead-acid batteries often have very short lifetimes in hot climate conditions, especially when they are occasionally fully discharged. Lead-acid batteries are also environmentally hazardous, since lead is a major component of lead-acid batteries and can cause serious environmental problems during manufacturing and disposal.
Flowing electrolyte batteries, such as zinc-bromine batteries, zinc-chlorine batteries, and vanadium flow batteries, offer a potential to overcome the above mentioned limitations of lead-acid batteries. In particular, the useful lifetime of flowing electrolyte batteries is not affected by deep discharge applications, and the energy to weight ratio of flowing electrolyte batteries is up to six times higher than that of lead-acid batteries.
However, manufacturing flowing electrolyte batteries can be more difficult than manufacturing lead-acid batteries. A flowing electrolyte battery, like a lead acid battery, comprises a stack of cells to produce a certain voltage higher than that of individual cells. But unlike a lead acid battery, cells in a flowing electrolyte battery are hydraulically connected through an electrolyte circulation path. This can be problematic as shunt currents can flow through the electrolyte circulation path from one series-connected cell to another causing energy losses and imbalances in the individual charge states of the cells. To prevent or reduce such shunt currents, flowing electrolyte batteries require sufficiently long electrolyte circulation paths between cells, thereby increasing electrical resistance between cells.
Another problem of flowing electrolyte batteries is a need for a uniform electrolyte flow rate in each cell in order to supply chemicals evenly inside the cells. To achieve a uniform flow rate through the cells, flowing electrolyte batteries define complex flow distribution zones. However, because electrolyte often has an oily, aqueous and gaseous multiphase nature, and because of structural constraints on the cells, uniform flow rates are often not achieved.
Another issue in these types of batteries where the battery employs an array of stacks of cells is that the stacks share a common flowing electrolyte. Since the stacks share the electrolyte, measurements of the open-circuit voltage across a stack only indicate whether the stack stores some non-zero amount of charge, rather than indicating the stack's state of charge relative to the other stacks in the system. Moreover, differences in the open circuit voltages between stacks are typically indicative of some internal abnormality that has altered a stack's internal resistance.
For example, in a zinc-bromine flowing electrolyte battery, the stacks share an aqueous zinc bromide electrolyte and have their own electrodes for deposit and dissolution of elemental zinc during charge and discharge cycles. In this type of battery, the electrolyte flow to a stack can be inhibited by poorly placed zinc deposits. Additionally, nucleation on the electrodes can cause dendrite formation and branching between cells. In either case, the internal resistance of the affected stack or the open-circuit voltage across the stack could be lowered.
Differences in open-circuit voltages between parallel-connected stacks in flowing electrolyte battery systems can affect the charge and discharge cycles of the stacks and, potentially, the operation of the battery. For example, in the aforementioned zinc-bromine battery, a lowered open circuit voltage in a particular stack causes an increase in the rate of zinc accumulation in the faulty stack during the charge cycle and a decrease in the rate of zinc reduction in the faulty stack during the discharge cycle. Moreover, the additional zinc stored in the faulty stack typically comes from the electrolyte normally utilized by neighboring stacks. As a result of the lowered zinc availability, the energy storage capacity of the neighboring stacks may be reduced. Another consequence is that the stack having the increased zinc accumulation does not fully deplete the zinc during discharge; eventually resulting in zinc accumulating on the electrodes of the faulty stack to such an extent that it causes internal short circuiting between the cells of the stack. This can potentially destroy the stack and possibly, the entire battery system. A further consequence is that the increased zinc accumulation can restrict the channels through which the electrolyte flows. As the electrolyte flow acts to cool the stack, the restricted flow may cause the stack to overheat.
In order to restore open-circuit voltages to a more uniform value, an equalization process may be executed. The equalization process includes fully “stripping”, i.e., fully discharging, each stack in the battery, completely removing any stored charge from all of the cells in all of the stacks. Ideally, this process eliminates the abnormality that initially caused the difference in open-circuit voltage between the stacks. For example, a full strip typically dissolves dendrites between plates and/or deposits obstructing electrolyte flow. However, a full strip of each of the cell stacks in the battery typically renders the battery entirely unavailable or available at a significantly reduced capacity for electrical applications, necessitating the purchase and installation of additional redundant battery systems. Moreover, a full strip is often unnecessary since typically a minority of the stacks in the battery is operating abnormally.
In addition, existing methods of stripping battery stacks in a flowing electrolyte battery are typically time consuming and may have to be repeated every few days for a recurring problem. When stripping, i.e., fully discharging, a cell stack, care must be taken to avoid cell reversal in which the polarity of one of the stacks becomes opposite the polarity of the other stacks. In such an instance, the cell stack with the reversed polarity becomes a load, drawing current from the other stacks. Thus, during discharge, a cell stack is first discharged to a low voltage level using a higher current. When the stack reaches the low voltage level, the magnitude of the current is reduced to slow the rate of discharge. As the voltage level continues to drop, the magnitude of current is repeatedly stepped down to reduce the rate of discharge as the voltage level approaches zero. By approaching the zero voltage level at a slow rate, discharge of the cell stack is discontinued when zero voltage is reached. While this stepped reduction in the discharge current avoids cell reversal, it is also a significant factor in the time required to strip the cell stacks in a battery.
Therefore, there is a need for an improved electrolyte flow battery design and methods and apparatus for controlling, monitoring, charging and/or discharging cells in a flowing electrolyte battery.